What Makes a Soap Bubble Rise and Fall

If a soap bubble does not burst too soon, it will begin to fall. Why?

To explain this, we must remember a balloon filled with hot air. It rises for some time and then falls again. A balloon rises because the hot air inside it is lighter than the surrounding air; and as it is lighter, it must rise. When the air inside a balloon cools, the weight of the balloon itself makes it fall.

A soap bubble is really a little balloon filled with hot air. The air that fills a soap bubble is warm air from our lungs. This air is much lighter than the air outside. It comes out of our lungs with such force that it can carry the weight of the water that makes up the skin of the soap bubble.

But this cannot last for a long time, because the skin of a soap bubble is very thin. So the heat inside a soap bubble soon escapes, the bubble becomes as cool as the air around it, and it begins to fall.

It is interesting to know that early experiments with balloons were based on soap bubbles.